Editorial Technical Reference

PSU voltage regulator module: DC-DC stages for the +5 V and +3.3 V rails of a computer power supply

Inside the desktop PSU · +5 V and +3.3 V DC-DC stages, ATX regulation and ripple limits, efficiency targets for the whole supply

This page covers the voltage regulator module inside a computer power supply unit: the DC-DC stage that turns the supply's +12 V into the regulated +5 V and +3.3 V rails delivered at the ATX connectors. It is judged by the PSU's output specification, not by a processor's needs. Ask three things first: what regulation and ripple the module holds at the connector, how much current each rail must carry, and how much loss the module may add to the whole supply's efficiency budget.

DC-DC voltage regulator modules and stages inside desktop and server power supply units that generate the +5 V and +3.3 V rails from +12 V. Multiphase VRM controllers, power stages and modules for processor, ASIC, FPGA and GPU core rails are covered on the Voltage regulator module (VRM) page. Complete AC-DC power supplies and isolated DC-DC bricks are outside this page.

CNFX editorial research · Sources checked October 12, 2026 · 5 sources

Hold the ATX rail limits Budget the module's loss

High-current DC-DC converter board with four wound inductors, a row of power transistors under a black finned heatsink, large electrolytic capacitors, a copper busbar and multi-pin wire harness connectors on a test bench — Multiphase DC-DC regulator board on a test bench

Start with your task

Hold the ATX rail limitsRegulation and ripple. Budget the module's lossEfficiency targets.

Hold the ATX regulation and ripple limits at the connector

Intel's ATX Version 3.x Multi Rail Desktop Platform Power Supply Design Guide (Table 4-2) requires +5 VDC within ±5% (4.75–5.25 V) and +3.3 VDC within ±5% (3.14–3.47 V), with tolerance required at all connectors. The +12 V rails are allowed +5%/−7% (11.20–12.60 V) to cover power excursions, and Intel notes that nominal 12 V may be set to 12.1 or 12.2 V for that reason. [1]

For ripple and noise (Table 4-6) the same guide sets 50 mV p-p maximum on +5 V, +3.3 V and +5 VSB and 120 mV p-p on the +12 V rails and −12 V, measured over 10 Hz to 20 MHz with a 20 MHz oscilloscope and with 0.1 µF ceramic and 10 µF electrolytic capacitors at the connector. [2]

Editorial calculation: 50 mV is about 1.5% of 3.3 V (0.05 ÷ 3.3) and 1.0% of 5 V, while the ±5% regulation window is ±165 mV on 3.3 V and ±250 mV on 5 V. A DC-DC module whose 12 V input sits anywhere from 11.20 to 12.60 V must hold both limits across that whole input range. [1] [2]

Give the supplier the input range from the 12 V bus, each output voltage and current, regulation and ripple limits at the connector, transient load steps, power-sequencing and protection requirements, cooling airflow, and the PSU form factor.

Budget the module's loss within the supply's efficiency targets

Intel's guide (Table 4-14) gives required minimum efficiencies at 115 V AC input of 70% at full load, 72% at typical (50%) load and 65% at light (20%) load, and notes that idle DC load for mainstream computers is 10 W and could go lower. [3]

ENERGY STAR Computers Version 9.0 (2025) sets internal power supply minimums of 80% at 10% load, 82% at 20%, 85% at 50% and 82% at 100% for supplies rated 500 W and below, with power factor 0.90 at 50% load for supplies of 75 W and above; for supplies above 500 W it sets 80%, 87%, 90% and 87%. [4]

Editorial calculation: with an assumed 95% efficient DC-DC stage delivering 3.3 V at 20 A (66 W), the stage dissipates 66 ÷ 0.95 − 66 ≈ 3.5 W of heat. Because the whole supply must meet the ENERGY STAR figures measured at its output, every watt lost in the DC-DC stage counts against them. [4]

Mornsun non-isolated regulators from 0.3 A to 100 A, and the data to request

Mornsun lists non-isolated switching regulators in three current classes: the K78 series from 0.3 A to 3 A, high-current modules from 0 A to 50 A and POL modules from 6 A to 100 A. [5]

Ask each supplier for efficiency curves at your input voltage, output ripple measured to the ATX method, load-step response and thermal derating with your airflow.

PWM controller, MOSFETs, output inductor and capacitors, sensing and heatsink

Part	Role	Fix in the order
PWM controller IC	Regulates output against the ATX window. [1]	Controller, frequency, protections.
Power MOSFETs	Switch the 12 V input.	Rating and package; heatsinking.
Output filter inductor	Stores energy each cycle; sets ripple current.	Inductance and saturation current.
Output filter capacitors	Keep ripple under 50 mV p-p. [2]	Type, capacitance, ESR.
Feedback and sensing	Holds voltage at the connector. [1]	Remote sense, if used.
Heatsink and airflow	Removes the stage's loss. [4]	Thermal derating curve.

Rail current, ripple, ±5% accuracy and 12 V input window for a +5 V / +3.3 V DC-DC stage

For a stage inside the PSU the input is the 12 V bus, not mains, so quote the input range as 11.20–12.60 V DC from ATX 3.x. [1]

Field	What it means and published values	Ask the supplier for
Output current rating (A)	Per rail; non-isolated modules listed from 0.3 A to 100 A. [5]	Continuous and peak current.
Switching frequency (kHz)	Sets filter size and loss.	Frequency and tolerance.
Output voltage ripple (mV p-p)	50 mV on +5 V and +3.3 V, 10 Hz–20 MHz. [2]	Ripple measured to that method.
Efficiency (%)	Whole supply: 85% at 50% load (≤500 W), 90% (>500 W). [4]	Module efficiency curve.
Operating temperature range (°C)	Ambient inside the PSU.	Range and derating.
Input voltage range (V AC)	For an internal DC-DC stage the input is the 12 V bus, 11.20–12.60 V in ATX 3.x. [1]	DC input range.
Output voltage accuracy (%)	±5% at the connector for +5 V and +3.3 V. [1]	Set-point accuracy.
Load regulation (%)	Change from light to full load within the ±5% window. [1]	Value.
Line regulation (%)	Change across the 12 V input range. [1]	Value.
Isolation voltage (V DC)	Non-isolated stages have none; isolation is in the main converter.	Confirm non-isolated design.
Operating humidity (% RH)	PSU environment.	Range.
Protection class	Over-current, over-voltage and short-circuit protection.	Protection list and limits.
Weight (kg)	Module mass.	Mass.

Send the rail, regulation and efficiency data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Input: [12 V bus range]
Outputs: [+5 V and +3.3 V; current each]
Regulation: [±5% at connector; ripple 50 mV p-p]
Transients: [load steps; slew rate]
Efficiency: [module curve; PSU target class]
Thermal: [airflow; ambient; derating]
Protection: [OCP; OVP; short circuit; sequencing]

See manufacturer records Send an enquiry through CNFX

Buyer questions

What regulation must the +3.3 V and +5 V rails hold?

Intel's ATX 3.x design guide requires ±5% at the connector: 3.14–3.47 V and 4.75–5.25 V. [1]

What ripple is allowed on the +3.3 V and +5 V rails?

50 mV p-p, measured from 10 Hz to 20 MHz with a 20 MHz oscilloscope. [2]

What efficiency does ENERGY STAR require of a computer power supply?

Version 9.0 sets 82%, 85% and 82% at 20%, 50% and 100% load for supplies of 500 W and below, and 87%, 90% and 87% above 500 W. [4]

Sources and what still needs a direct check

Sources checked 2026-10-12. Intel publishes the ATX power supply design guide that PSU makers design to; ENERGY STAR (US EPA) sets efficiency criteria for computer power supplies; Mornsun is a maker listing its own non-isolated DC-DC modules.

Intel — ATX Version 3.x Multi Rail Desktop Platform Power Supply Design Guide: DC voltage regulation (Table 4-2) ↗ Platform design guide Return to text ↑
Intel — ATX Version 3.x Power Supply Design Guide: output ripple and noise (Table 4-6) ↗ Platform design guide Return to text ↑
Intel — ATX Version 3.x Power Supply Design Guide: overall power supply efficiency levels (Table 4-14) ↗ Platform design guide Return to text ↑
ENERGY STAR — Program Requirements for Computers, Version 9.0 Final Specification (2025) ↗ Government programme specification Return to text ↑
Mornsun — Non-isolated switching regulators (K78, high-current and POL series) ↗ Maker product page Return to text ↑

Still to check yourself

Check ±5% regulation at the connector.
Measure ripple to the ATX method.
Budget module loss within the PSU efficiency target.
Confirm the 12 V input range.
Verify the legal supplier and factory.